Name ____________________________________
Period ____________
8-1 Describing Chemical Reactions
1. What are chemical reactions?
a. Chemical reaction –
i. Because chemical bonds store energy, when they are broken that energy is
released
ii. When the energy is released it is converted from chemical energy into one of the other
forms of energy:
1. Energy released as light
2. Energy released as Heat
3. Production of sound
4. Absorption or released of electrical energy
iii. When new chemical bonds form the reaction makes a new substance (product) that can
also be observed:
1. New products form as a gas
2. New products form as a precipitate
3. New products cause a color change
4. New products cause a change in oder
2. What are the mechanics of a chemical reaction?
a. Chemical reactions are like recipes, you add ingredients together to get a product.
b. Chemists report the “recipes” of chemical reactions in the form of chemical equations:
i. Chemical equation - an expression that uses symbols and chemical formulas to
describe a chemical reaction.
c. Reactants – the starting materials of a chemical reaction
d. Products – the ending materials of a chemical reaction
i. The most basic form of a chemical equation is:
Reactant(s)  Products
Where the arrow means yields or produces
ii. Many reactions occur to completion, which means that virtually all of the
reactants are turned into products
1. For instance when baking soda is added to vinegar, the reaction will keep going
until there is no more baking soda or vinegar left
iii. Some reactions don’t occur to completion, which means that the
reaction stops leaving both reactants and products
1. For instance when hydrogen gas (H2) is added to Nitrogen gas (N2) to produce
ammonia gas (NH3) the reaction stops before all the nitrogen and oxygen can bind
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together. As a result there is both N2, H2, and NH3 in the reaction chamber
afterwards
e. Chemists use symbols above the yield sign represent conditions necessary for a reaction to
proceed.
i.
means that the reactants must be heated in order to turn into products
ii.
means that the reactants must be electrified in order to turn into products
f. Chemists use symbols after the reactants and products to represent their phase
i. Liquid = (l)
ii. Gas = (g)
iii. Solid = (s)
iv.
Crystal = (cr)
v. Aqueous = (aq) (the substances are dissolved in water)
How would you read this reaction:
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8-2 Balancing Equations
1. How are equations used in chemistry?
a. All chemical equations must obey the law of Conservation of mass. (mass can
neither be created or destroyed)
b. The problem is that when some equations are written they describe the formation of products that
have fewer atoms than the reactants.
i. H2(g) + O2(g)  H2O : This equations says that you start with 4 and you end with 3. It
would appear to imply that every time a water molecule is produced 1 Oxygen atom
David Blain’s out of existence
In a chemical reaction:
The number of atoms in the reactants (mass of the reactants)
- must equal The number of atoms in the products (mass of the products)
ii. This disappearing act is kept from occurring by balancing equations
iii. Balancing equations– placing coefficients in chemical equations that make
sure the number of atoms of each element is the same at the start of the reaction
than at the end of the reaction.
1. Subscript – indicates number of atoms of an element present in a
compound.
A. Ex: H2O = 2 hydrogen atoms and 1 oxygen atom
2. Coefficient – indicates the number of molecules that take place in the
reaction
A. Coefficients multiplies the number of each atom in the molecule but keeps
the molecular identity the same
B. Ex: 2H2O = 2 water molecules each consisting of 2 hydrogen atoms and 1
oxygen atom for a total of 4 hydrogen atoms and 2 oxygen atoms
3. To balance a chemical reaction you can never change the
subscripts because that would change reactants and products that go into
and come out of the reaction. You can only add whole molecules that go into and
come out of chemical reactions using coefficients
H2(g) + O2(g)  H2O
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Is NOT balanced if you where to change the subscripts:
H2(g) + O2(g)  H2O2
this equation now reads that hydrogen gas and oxygen gas react to form peroxide that you use to clean out a cut,
not water that you drink
Is balanced if you where to change the number of molecules that go into and out of the reaction:
2H2(g) + O2(g)  2H2O
This equation reads that two hydrogen molecules and 1 oxygen
molecule react to form 2 water molecules
4
Start
H atoms
2 O atoms
End
4H
atoms
2O
atoms
2. How do you balance chemical reactions?
i. Steps to balance equations:
1. Write equation with symbols.
2. Count # of atoms on each side of the reaction.
3. Balance each atom separately by changing coefficients not subscripts
A. Make sure to count atoms that are even if they aren’t in the same molecule
i. IE: CH4 + O2  CO2 + H2O (you would count 3 oxygen atoms
total on the products side not 2 and 1)
4. At the end recheck every atom to make sure that the number you start with is the
number you end with
Balance the following equations:
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H2
+
P
NaNO3
C8H18
+
+
N2
→
O2
→
O2
→
NaNO2
→
NH3
P2O5
+
CO2
O2
+
H2O
8-3 Classifying Chemical Reactions
1. How are chemical reactions classified?
a. Depending on the reactants that go into a chemical reaction and the products that come out of a
chemical reaction, most chemical reactions can be classified as one of 5 different types:
i. Combustion reactions
ii.
iii.
iv.
v.
Synthesis reactions
Decomposition reactions
Single replacement reactions
Double replacement reactions
1. By determining what type of chemical reaction is taking place, you can predict the
products that will be formed
2. What are Combustion Reactions?
a. Combustion reaction - One substance containing
to produce
water
carbon
reacts with oxygen
and carbon dioxide
i. Combustion reactions usually involve expositions and flames
1. Explosions are involved because combustion reactions involve solids or liquids of
relatively low volume rapidly being converted into gas which has a
relatively larger volume
2. When there isn’t enough oxygen, the reaction does not go to completion and CO
and or C (soot) are produced
3. Your body gets its energy from a combustion reaction of sugar,
A. this is why you must breath in oxygen, and breath out CO2
B. Also the heat that is produced is what allows you to be warm
blooded
ii. How to balance combustion reactions:
1. Balance carbon first
2. Next balance the hydrogen making sure that an EVEN coefficient is placed with
the water
3. Finally balance the oxygen
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Balance these combustion reactions:
C2H6
+ O2
C4H6
→ CO2 + H2O
+ O2
→
C6H12O6 + O2 →
3. What are Synthesis Reactions?
a. Synthesis reaction – reaction in which two or more substances combine to form a
new compound
i. synthesis means to put together
ii. The basic form of a synthesis reaction is:
A
Fe
CaO
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+
B → AB
+
S → FeS
+ H2O → Ca(OH)2
Balance these synthesis reactions:
H2 + N2  NH3
Na + Cl2 
Iron (III) + Oxygen gas 
4. What are Decomposition Reactions?
a. Decomposition reactions – A reaction in which a single compound breaks down to
form two or more similar compounds
i. decompose means to break apart
ii. decomposition reactions are the opposite of synthesis reactions
1. The basic form of a decomposition reaction is:
AB → A
+
2HgO → 2Hg
+
CaCO3 → CaO +
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B
O2
CO2
5. What are single replacement reactions?
a. Singe replacement reactions – a single more reactive element reacts with a
compound to displace a less reactive element
i. The activity series is a way of arranging elements (usually metals) in order from greatest
to least reactivity.
ii. An activity series can be used to predict whether a single
replacement reaction will occur or not.
1. A more reactive element will replace a less reactive element.
2. A less reactive element will not replace a more reactive element
More Reactive
Less Reactive
iii. The general form of a single replacement reaction is:
A
Fe
3Mg
Ag
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+
+
+
+
BX
CuSO4
2Al(NO3)3
Pb(NO3)2
→
AX
+ B
→
FeSO4
+ Cu
→ 3Mg(NO3)2 + 2Al
→
No RXN
Balance these singe replacement reactions:
Na +

H2O
Na2O + H2
Li
+
NaCl 
Cu
+
ZnSO4 
6. What are Double-Replacement Reactions?
a. Double replacement reactions – a reaction where 2
products form by
exchanging atoms
i. The general from of a double replacement reaction is:
AX
+
BY
→
AY
+
CaCO3 + 2HCl
→
CaCl2 +
NaOH +
H2SO4 →
HOH +
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BX
H2CO3
Na2SO4
Balance these Double replacement reactions:
CoCl2 +
Na2O
FeO
+
CuO +
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
CoO + NaCl
Al(NO3)3 
ZnSO4 
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